The present invention generally is directed to pistons. More specifically, the invention is directed to pistons that employ cup seals that are attached to the piston head. Additionally, the present invention is directed to pistons made of a plastic material.
Wobble-type pistons are known as are cup seals. Additionally, it is known to construct wobble-type pistons employing two-piece rods. Further, it is known to make pistons out of plastic.
In U.S. Pat. No. 4,730,550, there is disclosed a two-piece piston rod construction wherein an upper portion of a piston rod is integrally formed with a piston of a wobble-type piston. The upper piston rod portion is secured to a lower piston rod portion by means of bolts that extend through the working surface of the piston, through the upper piston rod portion, and into the lower piston rod portion.
Current practice in the assembly of cup seals and pistons is to clamp a cup seal onto an upper surface of a piston by securing the cup seal between the piston and a disk. A typical example is disclosed in U.S. Pat. No. 4,730,550.
Because the cup seals are assembled down onto the piston, piston stack height control is frustrated due to variations in the clamping of the disk securing the cup seal to the piston. Additionally, the various cup seals can exhibit different compression characteristics resulting in different piston heights.